This invention relates to communication by means of optical fibers, and, more particularly, to a novel optical fiber master/slave transmission system to provide main traffic or trunking connections from higher echelon units such as central telephone communications offices to lower echelon units such as switchboards and/or substations. The invention utilizes a single optical fiber connecting means for transmission in both directions through the utilization of wavelength division multiplexing to achieve separation of traffic in the two directions.
The advantages of communicating by means of modulated light transmitted through optical fibers are well known and the techniques of optical fiber transmission have contributed significantly to the advance in telephonic communications. An optical fiber telephone communication system, for example, exhibits a lower attenuation than a comparable system which transmits modulated electrical signals over a wire or cable system, and hence fewer repeaters or booster type units in the lines are required with an optical system than with the predecessor wire systems. Also, optical systems have been shown clearly to have significant security advantages which make them highly desirable in military communications applications. Optical fibers are extremely difficult to tap into in order to discern messages being carried, and since they contain no metal and carry no electrical current, they cannot be detected by such things as metal detectors or radar. In addition, and of great significance to military personnel in the field, is that optical fiber systems and optical telephones and related equipment can be made entirely of lightweight inexpensive plastic materials. Optical systems are not subject to electrical interference caused by such things as atmospheric static, sun spots, or the electromagnetic pulses due to nuclear explosions or to jamming signals.
While optical communication systems of relatively basic and simple sorts have been known down through the ages, witness for instance historically, smoke signals, semaphors used in military and similar communications, illuminated symbols, photo phones and transmission of intelligence by light emitting diodes it has only been relatively recently that the development of thin, lightweight optical fibers and the development of light generators capable of producing laser beams that the practical applications of optical fiber communication such as is involved in this invention, have developed to a useful and practical state in the art. Known fiber optic communications systems usually utilize the optical fiber as the transmission medium only, with the terminal equipment at either end of the fiber optic link being installed as conventional electrical or electronic telephonic equipment, e.g., electrical telephones, teletypes, etc. Such systems require electro-optic interface units at each end of the fiber optic link in order to provide for translation of the optical signals into the conventional electrical signals utilized by known and conventionally available equipment.
In my co-pending U.S. application Ser. No. 856,859, filed May 21, 1986 as a File Wrapper Continuation of my earlier U.S. application Ser. No. 647,767 bearing the same title and filed Sept. 6, 1984, there is disclosed a hybrid telephone system comprising a plurality of optical telephones and a plurality of electrical telephones all connected to a conventional central office. In that invention disclosure, the optical telephones are taught to be provided with opto-electrical interface units colocated with the central office for converting optical signals received from the optical telephones to electrical telephone signals and, conversely, for converting electrical telephone signals from the central office signals which are received to optical signals for transmission to the optical telephones. This application is presently on appeal in the United States Patent and Trademark Office before the Board of Appeals, the latest official paper being the Examiner's answer dated Aug. 7, 1987. The teaching in that application is incorporated herein by reference, since the basic details of fiber optic telephonic communication working in practical consort cooperatively with electrical or electronic telephonic communications systems are explained in that application to the extent that the teaching there is considered to be classical and the application to be omnibus and/or bellwether in nature.
In earlier techniques in the art involving optical fiber telephone communications, the optical telephones disclosed utilize two or more optical fibers for communicating with a central office and/or with each or with a switchboard. The improvement comprehended by my said earlier invention involved the all optical telephone utilization of a single optical fiber capable of using two different wavelengths for two voice or traffic signals. Such wavelength multiplexing prevents cross-talk caused by reflections such as occurred with prior art telephone systems known up to that time.